For example, a semiconductor manufacturing process has a step of heat-treating a semiconductor wafer as a substrate. Recently, a heat treatment apparatus which batch-treats a large number of wafers is used in order to effectively achieve required heat treatment. In such a heat treatment apparatus, usually, a wafer boat made of quartz and mounting a large number of wafers thereon is accommodated in a heat treatment chamber, and the wafers are heat-treated in this heat treatment chamber. Regarding this heat treatment, first, the wafers accommodated in a wafer carrier made of a resin, e.g., Teflon, are transferred therefrom onto the wafer boat to be subjected to heat treatment. After predetermined heat treatment is performed, the wafers are transferred from the wafer boat to the wafer carrier.
This wafer transfer operation is performed by a transfer apparatus arranged between the wafer carrier and the wafer boat. In this transfer apparatus, the transfer apparatus main body is moved in the vertical direction and the angular direction in accordance with an operation pattern predetermined by the control mechanism so that it is located at a transfer operation position. Also, a wafer carrying fork serving as a treatment target carrying member arranged in the apparatus main body is moved in the horizontal back-and-forth direction. The wafers are transferred between the wafer carrier and the wafer boat by these operations.
In the actual transfer operation, the fork of the transfer apparatus must perform wafer placing and extracting operations while maintaining an appropriate positional relationship with the target position of the arranged substrate holding member, e.g., the carrier or wafer boat, or with the wafer to be transferred.
The position where the wafer carrier or wafer boat is placed, the posture with which the wafer carrier or wafer boat is placed, and the state of the wafer support grooves are not always the same. More specifically, sometimes the wafer carrier or wafer boat is placed in an inclined state, and various types of displacements may occur in the wafer carrier or wafer boat itself. For example, in the wafer boat, thermal deformation can occur in the heat treatment step. Deformation or strain may occur in the wafer boat due to cleaning performed in order to remove contaminants that attach to the wafer boat during heat treatment. Furthermore, a wafer boat having a specific displacement in itself may be replaced. In the carrier, deformation tends to occur entirely or partly since the carrier is made of a resin.
When various types of these displacements occur in the wafer support member, or when the wafer support state of the wafer support member is inappropriate, the operation position of the fork of the transfer apparatus controlled in accordance with the basic operation pattern is actually relatively displaced from the wafer to be transferred, and the aimed transfer operation cannot be sufficiently performed accordingly. For example, in the wafer extracting operation, sometimes the target wafer cannot be placed on the fork in the aimed state. In the wafer placing operation, sometimes the wafer cannot be accurately inserted, or cannot be inserted at all, in the support groove of the wafer support member.
From these reasons, when a carrier or wafer boat is newly arranged, an operation called teaching is performed so that a transfer operation, which matches it, is performed.
This teaching is an operation of presetting in the apparatus the coordinate values of the respective wafer grooves of the wafer boat and the coordinate values of the wafer support portion of the corresponding carrier. The basic operation pattern of the transfer apparatus is adjusted based on this teaching.
In this case, the groove width of the wafer boat is 3 mm, the thickness of the wafer is 1 mm, and accordingly, the clearance above and below the wafer is as very small as 1 mm. Also, the wafer and the grooves must be absolutely prevented from rubbing with each other. Therefore, high precision of .+-.0.1 mm is required in teaching.
Since the number of wafers mounted on the wafer boat is as large as 125 to 180, the number of times of operations in one teaching is very large. Also, teaching must be done again every time the boat is thermally deformed or cleaned by the process, leading to a high operation frequency.
This teaching operation is conventionally performed with the visual observation of the operator, and this operation is repeated several times until data having allowable precision is obtained. Even if, however, a certain precision is obtained through repeated operations, as it is done with visual confirmation, it is substantially impossible to realize the required high precision of .+-.0.1 mm described above. The teaching operation takes time since it is performed in a trial-and-error manner by actually moving the fork little by little while confirming the state through visual observation, leading to a long down time of the apparatus, which is inefficient. Since the result of teaching largely depends on the skill of the operator in charge of teaching and the like, the reproducibility of teaching is low. Further, the position of the wafer mounted on the wafer boat must be recognized accurately. However, a semiconductor wafer processing apparatus, e.g., a heat treatment apparatus, usually has a very small inner space. In confirming a portion in the apparatus that needs checking, the operator cannot but do so in an uncomfortable position. The working environment is poor.
From these reasons, auto-teaching which is done without requiring the operator is developed. As an example of auto-teaching, for example, a technique is known with which a position detection hole is formed in the groove of the wafer boat, the position of the hole is detected by an optical sensor to obtain the position of the boat, and Z-direction positioning that requires particularly high precision is performed.
However, since the boat is generally made of quartz, it is difficult to detect the boat with an optical sensor, so that stable detection cannot be performed. If the light transmittance or reflectance of quartz changes depending on cleaning or the process, or if the boat thermally deforms, auto-teaching cannot cope with this deformation. Furthermore, a large machining cost is required to fabricate such a boat.
In the transfer apparatus for automatically transferring the wafer, a sensor for detecting the presence/absence of the wafer to be transferred is required from a viewpoint different from that in teaching. More specifically, when the fork of the transfer apparatus is inserted in, e.g., the wafer boat, whether a wafer is present above the fork must be detected.
The heat treatment apparatus described above processes expensive wafers, and the wafer boat is also expensive. Accordingly, occurrence of accidents must be prevented. When, however, the distance between the wafer and the wafer groove of the boat is inappropriate, the fork may abut against the boat to push or pull it down, to rub the wafer against the groove portion of the boat, or to break the wafer or fork. A counter-measure that prevents occurrence of accidents is required.